German Patent No. 42 41 045 describes a method of anisotropic etching of silicon in structures preferably defined with an etching mask, in particular recesses with a precise lateral extent in silicon by using a plasma, with the anisotropic etching operation being carried out separately in alternating separate etching and polymerization steps which are controlled independently of one another, and with a polymer being applied to the lateral border of the structures defined by the etching mask during the polymerization step, being removed again partially during the following etching step and then being redeposited in the deeper parts of the side wall newly formed by the etching reaction, thereby producing local edge protection and local anisotropy.
Sulfur hexafluoride SF6 is used preferably as a gaseous etchant supplying fluorine in alternation with a passivation gas of trifluoromethane CHF3, which forms Teflon(copyright)-like polymers, in a high density plasma, e.g., with inductive excitation (ICP=inductively coupled plasma) or with microwave excitation (PIE=propagation ion etching), to etch silicon anisotropically at very high rates. High-rate etching is understood to refer to rates of 5 xcexcm/min, for example.
At the same time, a very high selectivity with respect to the photoresist mask layer, for example, can be achieved here. During the deposition steps, a side wall polymer is deposited on the side walls of etched structures. During the etching steps, etching is continued essentially isotropically in the silicon, resulting in effective protection of the newly exposed side walls locally during continued etching due to removal and redeposition of the side wall polymer material into deeper portions of the etching pit. Due to this entrainment of the side wall polymer toward deeper portions of the etching pit during continued etching, a local anisotropy of the essentially isotropic (because it is based on fluorine) etching process is achieved. Removal and redeposition are then achieved by the portion of ions accelerated toward a wafer, for example, due to the bias voltage applied during the etching cycles during the etching step, but not striking it exactly perpendicularly and therefore not striking the etching base directly but instead striking the side walls of the structure, pushing the side wall polymer deeper there. This portion of ions not striking exactly perpendicularly must therefore be optimized for the process management, and the ion energy must also be selected on the basis of the high frequency power responsible for the bias voltage of the substrates to the plasma so that a perpendicular wall profile with minimal wall roughness is achieved.
In a preferred embodiment of the method described in German Patent No. 42 41 045, for example, a pressure of 13.3 xcexcbar is selected for the deposition cycle and a pressure of 26.6/xcexcbar is selected for the etching cycle. To adjust the pressures in accordance with these specifications, for example, a slow pressure regulator may be set at a setpoint pressure of 19.95 xcexcbar. Because of the different effective gas flow rates during the deposition and etching cycles, a pressure of approximately 13.3 xcexcbar is then achieved automatically in the deposition step. The deposition process is the most efficient at this pressure. A pressure of approximately 26.6/xcexcbar is automatically achieved in the etching step, which is a favorable value for the etching step. A fast pressure regulator with two independent setpoint settings may also be used to make it possible to set the deposition step pressure and the etching step pressure at the selected values.
However, the following problem occurs with this known method. The reaction releasing the fluorine radicals necessary for the etching step is the decomposition of SF6 into SF4 according to the following equation:       SF    6    ⇄            SF      4        +          2      ⁢      F      
Since the equilibrium of this reaction is on the side of the free fluorine radicals under the reaction conditions, a large quantity of these radicals are available. Therefore, even at a very high excitation density in the plasma, with several times 10% of the SF6 feedstock being converted in the manner described here, additives which inhibit the reverse reaction are unnecessary. However, a relatively small amount of the SF6 is converted beyond SF4 to sulfur fluoride compounds with an even lower fluorine content or even to sulfur atoms.
For the etching process described here, the occurrence of sulfur is of no relevance because deposition of sulfur cannot occur on the wafer surface or in the process chamber at the stated process pressures at a substrate temperature of 30xc2x0 C. or even at lower substrate temperatures of xe2x88x9230xc2x0 C., for example. However, sulfur deposits which have an extremely negative effect are formed in particular on unheated walls in the area of the vacuum pumps, in particular turbomolecular pumps, and in the exhaust gas line of the turbo pumps, where the gas pressure is increased from the process pressure to the back pressure of the turbo pump. Formation of sulfur is based on the usual reaction of sulfur atoms to form the known sulfurous ring molecules, in particular S8. This contamination of the exhaust gas lines and parts of the turbo pump with sulfur deposits can lead to failure of the turbo pump in the medium-term, but it also entails the risk that larger quantities of loose sulfur dust might be entrained inadvertently into the process chamber when the vacuum pumps are turned off or if they fail, for example.